This invention relates to a hydraulic booster comprising a booster piston, a pressure-control device, an annular piston bearing against the booster piston, and a central piston, which is movable relative to the annular piston by means of a brake pedal and actuates the pressure-control device, the central piston and the annular piston being adapted to be coupled together after the force of a spring is overcome, whereupon they are movable as one.
Such a booster is disclosed in the copending U.S. application of W. Volkmar and H. Steffes, Ser. No. 67,269, filed Aug. 17, 1979, assigned to the same assignee as the present application.
In the above-cited copending application a hydraulic booster is described which includes a booster piston, a central piston and an annular piston. In this arrangement the annular piston is movable within limits toward the central piston, the central piston acts as a control spool in the booster piston and together with the annular piston as a pressure-control device. The central piston and the brake pedal are directly mechanically interconnected. In such a device, which provides a hydraulic booster having a so-called two-stage action, the pressurized fluid subjected to pressure is led into the pressure chamber, thus, the pressure will inevitably be transmitted via the central piston to the brake pedal. It is, however, the intention to keep the hydraulic reaction on the brake pedal as small as possible, i.e. to attain a two-stage action as rapidly as possible. Due to the construction of the booster of the above-cited copending application, the hydraulic reaction is only limited to some minimum value, which is predetermined by the diameter of the central piston. The diameter of the central piston may only be reduced to a certain size, since it has to accommodate the channels for the pressure control. Therefore, the hydraulic reaction is not as small as possible.